1. Field of the Invention
The present invention relates generally to a automatic automotive transmission and more specifically to a line pressure control arrangement which adjusts the pressure level in accordance with the variation in the inertia phase during no load upshifting.
2. Description of the Prior Art
An automatic transmission Service Manual (publication A261C07) issued in March 1987 relating to the so called RE4RO1A type transmission, describes an arrangement which utilizes tabled data for controlling the line pressure during shifting and non-shifting modes of transmission operation. In this arrangement the engine throttle valve opening degree is used to determine the duty cycle of a signal used to control a line pressure control solenoid valve.
In the above mentioned arrangement, because the line pressure is controlled by a single throttle valve position (engine load) -- line pressure schedule, during shifting operations it has been found very difficult to successfully control the line pressure to just the right level for all of the various shift possibilities. For example, the line pressure level required during a power ON shift is greater than in the case of a corresponding power OFF shift made at the same speed. Further, if the shift occurs at a high vehicle speed the required level of line pressure is higher than in the case the same or similar shift occurs at low speeds even though the position of the throttle valve may be the same.
Accordingly, as shown in FIG. 9, with the above type of transmission, when the accelerator pedal is released and the engine throttle valve is allowed to close, the load on the engine suddenly drops, the level of the line pressure is lowered and the throttle valve opening/vehicle speed co. ordinate crosses the upshift line in the manner indicated by the arrow.
Under these conditions, when the vehicle speed is relatively high, the relative rotation between the driven and driving elements of the friction element or elements which are engaged in response to the upshifting, is relatively high and the line pressure produced in accordance with the instant throttle setting tends to be inadequate to induce the required amount of engagement. On the other hand, when the vehicle speed is low, the relative rotation between the driving and driven elements is small and shift shock tends to be induced as the pressure which is applied tends to be overly high for the instant set of operating conditions. Setting the pressure at a level suitable for one of course invites problems with the other.